1. Field of the Invention
The invention relates to a hybrid electric vehicle powertrain having an internal combustion engine and an electric motor/generator and battery sub-system, which are independent power sources for powering vehicle traction wheels. At least one power flow path is established between each power source and the traction wheels through transmission gearing.
2. Background Art and Discussion of a Known Hybrid Powertrain
U.S. Pat. Nos. 6,407,521 and 6,427,794 disclose hybrid vehicle powertrains having a motor/generator and an internal combustion engine, which function in cooperation with a planetary gear unit to define multiple power flow paths. The engine and the motor/generator provide parallel power delivery through the gear unit during one operating mode. The motor/generator can be used to establish a power flow path independently of the engine.
U.S. Pat. No. 6,176,808 discloses a hybrid powertrain wherein an engine and an induction motor act in parallel power flow paths. The induction motor provides torque to complement engine torque during vehicle acceleration. It also can be clutched to the engine crankshaft to permit engine starting.
In the copending patent application identified above, the disclosure of which is incorporated herein by reference, the planetary gear unit provides a mechanical power flow path between a generator and an engine. A separate motor acts as a secondary power source. The motor may act as a series power source or it may function in cooperation with the engine to establish split power delivery paths through the planetary gear unit. Further, the generator may function as a motor in a torque flow path through the planetary gear unit.
In the case of the hybrid transmission of the copending patent application, the planetary ring gear is connected directly to the output torque delivery shaft through a set of fixed ratio gears. This makes it possible for torque to be distributed from the planetary gear set to the vehicle traction wheels during forward drive. To establish reverse drive, it is necessary to use the electric motor as a torque source since the engine is effective to deliver driving torque through the planetary gearing only in the forward drive mode.
If the state-of-charge of the battery during reverse drive is below a desired level, operation in reverse drive must be severely restricted in order to protect the battery. If an attempt is made to charge the battery by operating the engine to create generator power for charging the battery, the reaction element of the planetary gear unit (which in this instance is the ring gear) would be subjected to reaction torque that must be overcome by the electric motor in order to achieve reverse drive. This limits reverse drive performance for the hybrid electric vehicle.